Hydraulically openable non-return valves are widely used in the hydraulic control of roof supports and other equipment in a mineral mining installation. Such a known device has a control piston which moves a valve stem against the restoring force of a spring to lift a valve closure member from a seating. With the valve opened a consumer line, leading for example to a prop working chamber, makes connection with a pressure line or a return line via a chamber in the device. When the valve is closed the hydraulic pressure in a chamber of the device, i.e. the consumer pressure, acts on the closure member so that the closure force of the spring is supplemented by this hydraulic pressure. As a result a high control pressure needs to be applied to the control piston to cause the valve to respond and open. The change over or switching control pressure necessary to open the valve is dependent on the prevailing pressure in the consumer line which can undergo considerable fluctuation. Thus the pressure needed to cause the valve to open is imprecise and can lead to erroneous change-over and malfunctioning of the equipment.
A general object of the present invention is to provide an improved valve device of the above-mentioned kind in which a more reliable operation even under wide variation in consumer pressure can be ensured.